The invention relates to a seismic system suited for being lowered into a well for the reception of acoustic or seismic waves, and more particularly to a seismic system for wells provided with a relatively great number of sensors distributed over a great length.
Systems suitable notably for the reception of acoustic or seismic waves for wells generally comprise one or several reception units connected to each other, lowered into a well at the end of a multiwire electro-carrying cable connected to a surface installation. Each of these reception units comprises a sealed enclosure associated with anchoring means enabling the enclosure to be applied against a portion of the well wall and one or several geophones, possibly triaxial, to be coupled with the surrounding geologic formations. The various sensors may be connected separately to the surface installation so as to transmit the signals they pick up. More frequently, they are connected to a signal acquisition device located in the reception unit at the top of the reception system. This device collects by multiplexing, digitizes and stores the signals coming from all the sensors of the reception system, before transmitting them in a coded form to a surface control and recording set.
Such reception systems are for example described in U.S. Pat. Nos. 4,428,422, 4,874,060, 4,862,425, 4,901,289 or French Patents FR 2,564,599 or 2,636,741 assigned to the same applicant.
Said known seismic systems implement seismic prospecting methods where seismic signals reflected by the discontinuities of a surveyed subsoil in response to the emission of seismic pulses in the subsoil by means of a source arranged at the surface, in another well or in the same well, are picked up simultaneously in a limited number of depths, and where the system is moved successively in a great number of depths along the well, so as to multiply for each sensor the number of reception locations.
The previous seismic systems may give satisfactory results provided that the well through which the array of sondes used is run is available long enough to perform seismic emission-reception cycles in all the successive locations selected and thereby compensate for the small number of sensors. This is a great handicap in wells of limited availability such as production wells.